To increase operating efficiencies of modern aircraft engines it is desirable to decrease weights of component parts. Substantial decreases in weights of components, such as propeller blades, have been achieved through use of composite materials, including for example graphite fiber reinforcements within an epoxy matrix. Composite blades, however, must be reinforced at their leading edges to provide adequate strength to protect the blade from erosion and foreign object damage, and especially from damage as a result of leading edge impact with birds, ice, stones, rain and other debris.
As is well known in propeller blade technology, adequate protection of a leading edge of a composite propeller blade is achieved by securing an electroformed nickel sheath to the leading edge. Manufacture of such electroformed sheaths is well known, as described for example in U.S. Pat. No. 4,950,375 to Leger, which Patent is hereby incorporated herein by reference. Typically, a die or mandrel, made of a conductive material such as titanium, is formed to have an exterior surface that conforms to a blade's airfoil configuration minus the thickness of the sheath to be electroformed on the mandrel. Desired thicknesses of the sheath are achieved by a well known process of "shielding", wherein barrier walls or shields are placed adjacent the mandrel in such positions that the shields direct the flow of an electroplate solution when the mandrels are placed in an electroplate bath. For example, where a sheath leading edge must be thicker, and hence stronger, than a sheath trailing edge, a shield portion adjacent a first surface section of the mandrel upon which is formed the sheath leading edge would be positioned a greater distance from that surface section of the mandrel than a shield portion adjacent a second surface section of the mandrel upon which is formed the sheath trailing edge. After the mandrel has been in the electroplate bath for a pre-determined length of time, it is removed; the electroformed sheath is next mechanically removed from the mandrel; and the sheath is then machined to smoothly fit over a composite component of the blade, in a manner well known in the art.
While electroformed sheaths have provided satisfactory protection for propeller blades, known sheaths are as yet inadequate for application to higher speed parts, such as fan blades of gas turbine engines. First stage fan blades of high bypass or advanced ducted engines in particular have become larger and travel at increasingly higher speeds to achieve desired performance requirements. Consequently the momentum of such blades is so great that blades made of composite materials having standard electroformed sheaths are not sufficiently strong to withstand ordinary foreign object damage within desired operational ranges of the blades. Therefore, most fan blades in modern gas turbine engines are manufactured of a hollowed-out metal, such as titanium, at extreme cost in labor and materials. The resulting all metal blades are much heavier than equivalent sized fan blades made of composite materials with an electroformed sheath.
Known electroformed sheaths are typically limited so that a ratio of the thickness of a thickest part of the sheath (e.g., the leading edge of the sheath) to the thickness of the thinnest part of the sheath (e.g., the trailing edge of the sheath) is generally 5:1, and may reach 10:1 at great cost. (The aforesaid ratio being hereinafter referred to as the "thickness range ratio".) For example, standard constraints of manufacture of the resulting blade require that the trailing edge of the sheath be no greater than 0.006 inches thick; which means 0.006 inches between an exterior surface of the trailing edge and an opposed inner surface of the trailing edge that contacts the composite material. Consequently, based on the thickness range ratio of existing electroformed sheaths, the leading edge can be no thicker than 0.030 to 0.060 inches. Appropriate strength requirements for electroformed sheaths on modern fan blades, however, mandate that the leading edge be approximately 0.500 inches thick, while manufacturing constraints require that the trailing edge remain at approximately 0.006 inches thick; being a thickness range ratio of approximately 80:1.
Accordingly, it is the general object of the present invention to provide an improved electroformed sheath that overcomes the strength, weight and cost problems of the prior art.
It is a more specific object to provide an electroformed sheath that affords adequate strength for protection of composite material components of a part such as a fan blade of a modern gas turbine engines.
It is a further specific object to provide an electroformed sheath for a part such as a fan blade that enhances the structural integrity of a leading edge of the blade.
It is another specific object to provide an electroformed sheath that facilitates affixation of the sheath to composite material components of a part.
It is yet another object to provide an electroformed sheath having a sufficiently thick leading edge to enable re-contouring or repair of the sheath to extend its useful life.
It is still another object to provide an electroformed sheath that may be fabricated by known manufacturing processes.
The above and other advantages of this invention will become more readily apparent when the following description is read in conjunction with the accompanying drawings.